Miniaturization and integration of optical systems analogous to advanced microelectronic technology has been hampered by fundamental and practical limitations. Key among these is diffraction, which limits the confinement of optical fields, and sizes of optical elements, to dimensions comparable to the optical wavelength. In addition, conventional optical components do not lend themselves well to modern thin film integration techniques. Integration of optical elements on a silicon chip analogous to microelectronics technology would have tremendous impact on both optoelectronics and microelectronics industries1.
Active Si-compatible components such as sources, modulators, switches and detectors are needed if a successful Si-based integrated optoelectronics technology is to be developed. Until recently, however, silicon was not considered a suitable material for optical devices. This is in part due to a low optical efficiency arising from its indirect bandgap and because unstrained silicon does not exhibit an electro-optic effect. Optically active compound semiconductors or electro-optic oxides have been the primary materials explored for electro-optic integration. Unfortunately, after years of development such systems remain costly to manufacture and low in yield. At the same time, there has been a great deal of recent activity demonstrating silicon based electro-optic components2. Some particularly exciting work has occurred in the area of silicon based optical modulators. Modulators are an essential “building block” component in any integrated optical circuit.
Because of the absence of an electro-optic effect, Si based modulators often rely on free carrier induced changes to the index of refraction and/or free carrier absorption. This approach has been used to develop electro-optical modulators using, for example, a Si rib waveguide with integrated bipolar mode FET,3 a lateral p-i-n diode with a Bragg reflector,4 and MOS capacitors with a Mach-Zehnder interferometer.5,6 The first of these relies on free carrier induced absorption. In the latter two approaches, a carrier-induced shift in the real part of the index of refraction of a silicon waveguide modulates the intensity. Increasing the free carrier concentration by 1019 cm−3 will reduce the index of refraction5,7 by about −0.01 and give an absorption coefficient8 of ˜100 cm−1. Carrier concentrations of this order were used by Intel to obtain 16 dB (84%) modulation in a Mach-Zehnder interferometer with integrated MOS capacitor phase shifters5 and more recently to demonstrate 10 Ghz bandwidth.6 
While devices such as these are interesting for specialty applications, their miniaturization and integration on Si in a manner analogous to advanced microelectronics technology is hampered by fundamental and practical limitations.9 Key among these is diffraction, which limits the confinement of optical fields, and sizes of optical elements, to dimensions comparable to the optical wavelength. Coupled with this is the small size and spatial extent of free carrier-induced effects. For these reasons, both the Bragg reflector and Mach-Zehnder interferometer require an active region several millimeters long. With Intel's Mach-Zehnder design, for example, the small voltage-induced index change is confined to a narrow layer (˜10 nm) adjacent to the oxide surface in an MOS capacitor. This means that only a small portion of the waveguide (and optical field) volume is modulated. In the bipolar field-effect transistor (FET) design, in contrast, reasonable modulation is observed in a 100 μm long modulation region. The power required to obtain sufficient free carrier absorption for modulation in this structure, however, leads to significant heating and/or voltages and currents that are incompatible with conventional Si electronic devices. Without some approach to concentrate optical fields into the region where free carrier densities are modulated, long interactions lengths, high injection current densities, and associated heating are common to most silicon-based modulator designs.10 